The present invention relates to a method of controlling an air conditioner, and more particularly, to a method of controlling a cooling operation of an air conditioner in a low outside air temperature environment.
In general, air conditioners have a refrigerating cycle including a compressor, a four-way valve, an outdoor heat exchanger, an expansion valve, and an indoor heat exchanger. In the winter in which the outside air temperature is low, generally, the air conditioner performs a heating operation to circulate a high-pressure and high-temperature refrigerant gas discharged from the compressor in the order of the four-way valve, the indoor heat exchanger, the expansion valve, the outdoor heat exchanger, the four-way valve, and the compressor, thereby using the indoor heat exchanger and the outdoor heat exchanger as a condenser and an evaporator, respectively.
On the other hand, in the summer in which the outside air temperature is high, generally, the air conditioner performs a cooling operation to circulate a high-pressure and high-temperature refrigerant gas discharged from the compressor in the order of the four-way valve, the outdoor heat exchanger, the expansion valve, the indoor heat exchanger, the four-way valve, and the compressor, thereby using the outdoor heat exchanger and the indoor heat exchanger as the condenser and the evaporator, respectively. In addition to this type of air conditioner, air conditioners having a dehumidifying function or a reheating and dehumidifying function are put on the market.
In recent years, particularly, a lot of heat sources, such as illuminators or electronic apparatuses, have been provided in the room, such as an office. Therefore, in many cases, the air conditioner is operated in a cooling mode even in a low outside air temperature environment in which the outside air temperature is equal to or smaller than, for example, 10° C.
In the cooling operation performed in the low outside air temperature environment, assuming that an indoor load is constant, when the outside air temperature falls, the refrigerant in the outdoor heat exchanger is excessively supercooled, and a refrigerant flows into the outdoor heat exchanger as a liquid refrigerant, which results in a reduction in the amount of refrigerant circulated. Therefore, it is necessary to open the expansion valve (in many cases, an electronic expansion valve).
In a case in which the outside air temperature falls to be lower than the freezing point, even when the expansion valve is fully opened, only a gas refrigerant is circulated, and the indoor heat exchanger is superheated, which makes it difficult to obtain desired cooling capability. In addition, the temperature around an inlet of the indoor heat exchanger is lower than the freezing point, and the inlet is frozen.
In order to solve this problem, it is necessary to reduce the number of rotations of the outdoor fan to prevent supercooling of the outdoor heat exchanger, and circulate the liquid refrigerant through the indoor heat exchanger. That is, in the cooling mode in the low outside air temperature environment, it is necessary to control the expansion value to be opened larger than that in a normal cooling mode, and control the outdoor fan to rotate at a lower rotational speed than that in the normal cooling mode.
In a method of controlling a cooling operation in the low outside air temperature environment according to the related art, for a predetermined number of rotations of the compressor determined by an indoor unit, when the outside air temperature falls, the number of rotations of the outdoor fan is reduced. Therefore, a target discharge temperature of the compressor is calculated using the outside air temperature, the temperature of the indoor heat exchanger, the number of rotations of the compressor, and the predetermined degree of superheat as variables. Then, the opening degree of the expansion valve is adjusted, on the basis of the difference between the calculated value and a detected discharge temperature, to control the amount of refrigerant circulated, thereby preventing the excessive superheating of the indoor heat exchanger (a discharge temperature control method).
Patent Document 1 discloses the following configuration: in a cooling mode in a low outside air temperature environment, in order to prevent the indoor heat exchanger from being frozen and to widen a cooling operation range, a set temperature is compared with the outside air temperature; and on the basis of the result of the comparison, when the operating frequency of the compressor is higher than a predetermined value, the expansion value is opened, but when the operating frequency of the compressor is lower than the predetermined value, the expansion value is closed.
Further, Patent Document 2 discloses a structure including: a first pressure difference adjusting apparatus that, in a cooling mode in an environment in which the outside air temperature is lower than a predetermined value, when the difference between the high pressure and the low pressure of a compressor is lower than a predetermined value, reduces the number of rotations of an outdoor fan to prevent the difference between the high pressure and the low pressure of the compressor from being reduced; and a second pressure difference adjusting apparatus that increases the operating frequency of the compressor when it is difficult to maintain the difference between the high pressure and the low pressure to be larger than a predetermined value by using only the first pressure difference adjusting apparatus.
[Patent Document 1]
    Unexamined Japanese Patent Application Publication No. Hei7-158980[Patent Document 2]    International Application Publication No. WO2003/083376
However, in the discharge temperature control method according to the related art, the same control method is used for a normal cooling mode (at the room temperature) and a cooling mode in a low outside air temperature environment. Therefore, when the outside air temperature falls, it is difficult to control the amount of refrigerant circulated due to the following factors.
(1) The number of rotations of the outdoor fan is reduced to increase a condensation temperature. Therefore, the detection of the outside air temperature is greatly affected by the temperature of the outdoor heat exchanger. (2) As the outside air temperature falls, the discharge temperature falls. Therefore, there is little variation in the amount of refrigerant circulated through the refrigerating cycle that is suitable when the above-mentioned variables vary and a target discharge temperature when these variables do not vary. (3) Since the amount of refrigerant circulated through the refrigerating cycle is reduced, the response of a discharge temperature detection value to a change in the variables is slow. (4) When an inlet of the indoor heat exchanger is superheated, a unit for detecting the superheating of the inlet is not provided.
When the above-mentioned factors (1) to (3) are combined with each other, outside air temperature that is higher than the actual outside air temperature is detected, so that a higher discharge temperature than a discharge temperature at which the optimum amount of refrigerant circulated is obtained is calculated. As a result, the expansion valve is closed to increase the discharge temperature up to a target value. Since the difference between the discharge temperature and the ordinary temperature is small, the expansion valve is narrowed. As a result, the amount of refrigerant to be circulated is considerably reduced.
As the detecting unit described in (4), generally, a temperature sensor is provided in the indoor heat exchanger to detect the temperature of the center of a pipeline passing through the indoor heat exchanger. Therefore, in order to detect the superheating of an inlet of the indoor heat exchanger, an additional temperature sensor needs to be provided in the vicinity of the inlet of the indoor heat exchanger.
In order to solve the above-mentioned problems, it is necessary to change the outside air temperature detecting method, improve a method of detecting and controlling the discharge temperature, and additionally provide a temperature sensor for monitoring the temperature of the inlet of the indoor heat exchanger. Therefore, the specifications and the design of the air conditioner according to the related art should be significantly changed.
An object of the invention is to provide a method of appropriately controlling the amount of refrigerant circulated in a cooling mode in a low outside air temperature environment without significantly changing the specifications and the design of the existing air conditioner.